Thermally-formable and cross-linkable precursor of a thermally conductive material

ABSTRACT

The invention relates to a thermally-formable and cross-linkable precursor of a thermally-conductive material comprising a) one or more crosslinkable polymers where the melt flow index of the polymer or mixture of polymers (measured at 190° C. according to ASTM D-1238), respectively, is 10-100 g/10 min and b) one or more thermally-conductive fillers in an amount of at least 60 wt. % of the total weight of the precursor.

FIELD OF THE INVENTION

The invention relates to a thermally-formable and cross-linkableprecursor of a thermally-conductive material and to thethermally-conductive material obtainable by cross-linking the precursor.The invention furthermore relates to a thermally-conductive adhesivetape comprising a backing and one or more adhesive layers wherein saidbacking comprises the thermally conductive material.

BACKGROUND

Thermally-conductive materials are known and are used, for example, forproviding a thermal bridge between printed circuit boards (PCBs) andheat sinks. Printed circuit boards generate heat under use conditions tothe extent that this heat must be diffused to allow continuous use ofthe PCB. Heat sinks in the form of metal blocks are commonly attached toPCBs to allow excess heat to be conducted away from the PCB and radiatedinto the atmosphere. Known thermally-conductive materials are based on,for example, gel masses, pads or greases that must be mechanicallyclamped between the PCB and heat sink.

More recently, thermally-conductive adhesive tapes comprising anadhesive material have been introduced. These thermally-conductive tapeshave the advantage that they form an adhesive bond with the twosubstrates to be connected and no mechanical clamping is required.Though heat-activatable adhesives can be used, pressure-sensitiveadhesives (PSAs) have the additional advantage that an adhesive bond isformed by simply pressing the assembly, comprising the substratessandwiching the adhesive material, together at room temperature withoutrequiring that the adhesive be activated with heat. Two approaches havebeen used for providing thermally-conductive adhesive tapes havingpressure-sensitive adhesive characteristics.

One approach for providing thermally-conductive adhesive tapes employs athermally-conductive adhesive material. This approach has the advantagethat only a single layer of such adhesive material is required toprovide an adhesive tape. One disadvantage, however, is thatthermally-conductive fillers must be added directly to the adhesivematerial and this may tend to reduce the quality of the adhesive bond.Adhesive tapes comprising no supportive and strengthening backingtypically also need to have a relatively high thickness and thickeradhesive tapes inherently provide a relatively high thermal resistancedue to the large distance over which the energy must be transmitted.Relatively thick, soft adhesive tapes are also difficult to cut intosmall discrete pieces by common converting techniques such asdie-cutting.

A second approach for providing thermally-conductive adhesive tapesemploys a polymeric film backing bearing separate layers of PSA on bothsurfaces. U.S. Pat. No. 6,165,612 (Bergquist) discloses a thermallyconductive, electrically insulative mounting pad to be positionedbetween a base surface of a heat-generating solid state electronicdevice and a mounting surface of a heat-sink; said mounting padcomprising a film consisting of polyphenylsulfone matrix impregnatedwith a thermally-conductive particulate filler in an amount ranging from10-50% by weight of polyphenylsulfone. The mounting pads optionallycomprise layers of adhesive on both sides.

U.S. Pat. No. 5,213,868 (Chomerics) discloses a thermally-conductivesupport material bearing thermally conductive pressure-sensitiveadhesive on both surfaces, where at least one exposed adhesive surfacehas embossments, grooves or channels to suppress air entrapment.

Japanese patent application JP 2000319454 discloses a flame-retardantadhesive tape comprising a backing of silane-crosslinked ethylene-basedpolymers. Thermal conductivity is not discussed in this reference.

Known commercially available adhesive tapes having a polymeric filmbacking include THERMATTACH™ T404 (available from Chomerics, Woburm,Mass./USA) which comprises a polyimide film backing and BONDPLY™ 660(available from The Bergquist Company, Edina, Minn./USA) which comprisesa polyethylenenapthalate (PEN)-based polymeric film backing.

This second approach for providing thermally-conductive adhesive tapeshas also proven to be technically viable and commercially successful,but polymeric film backings that provide acceptable thermal andelectrical properties and are suitable for use in such tapes can bedifficult to manufacture and are often high in cost.

It is an object of the invention to provide new thermally-conductiveadhesive tapes which have an improved combination of thermal, electricaland adhesive properties and are obtainable by simple and cost-effectivemanufacturing methods. Another object of the invention is to provide newthermally-conductive materials which have an advantageous thermalstability, a high breakdown voltage and an advantageous thermalconductivity. It is another object of the invention to provide newthermally-conductive materials suitable for use as a backing orthermally-conductive tapes. Other objects of the invention can readilybe taken from the following detailed specification.

BRIEF SUMMARY OF THE INVENTION

The invention describes a thermally-formable and crosslinkable precursorof a thermally-conductive material comprising a) one or morecrosslinkable polymers where the melt flow index of the polymer orpolymers (measured at 190° C. according to ASTM D-1238), respectively,is 10-100 g/10 min and b) one or more thermally-conductive fillers in anamount of at least 60 wt. % of the total weight of the precursor.

The invention also refers to a method of manufacturing the precursorcomprising the steps of: a) providing one or more crosslinkable polymerswhere the melt flow index of the polymer or mixture of polymers,respectively, is 10-100 g/10 min (as measured at 190° C. according toASTMD-1238) and b) compounding the polymer or mixture of polymers withone or more thermally-conductive fillers in an amount of at least 60 wt.% based on the total weight of the precursor.

The invention also describes a method of providing the precursorcomprising the steps of: a) providing one or more polymers where thepolymer or mixture of polymers, respectively, has a melt flow index of10-100 g/10 min (as measured at 190° C. according to ASTMD-1238) andwherein at least one of the polymers has an ethylene unit content of atleast 30% by weight, b) reacting the polymer with a vinyl silane of theformula RR′SiY₂ (I), wherein R is a monovalently olefinicallyunsaturated radical, R′ is a monovalent radical free of aliphaticunsaturation and Y is a hydolyzable organic radical, and a free-radicalinitiator in a heated mixing device to produce a moisture-curablepolymer and c) compounding the moisture-curable polymer with one or morethermally-conductive fillers in an amount of at least 60 wt. % of thetotal weight of the precursor in a heated mixing device. The inventionalso refers to a method of manufacturing a shaped thermally-conductivematerial comprising the steps of: a) providing the precursor of presentinvention, b) thermally forming the precursor to a desired shape and c)crosslinking the precursor.

The invention also describes a thermally-conductive material obtainableby the method of the invention.

The invention also describes an adhesive tape comprising at least a filmbacking bearing an adhesive layer on at least one of the major surfacesof the film backing, wherein the film backing is obtainable by extrudingthe precursor into the shape of a film and crosslinking the film.

The invention also refers to the use of the adhesive tape of the presentinvention for providing thermal conductivity between two substrates.

Furthermore, the invention describes an assembly comprising the adhesivetape in a bonding relationship between two substrates.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 shows the changes in the elastic torque, S′, of the precursor ofExamples 3 and 4 of the present invention while exposed to a temperatureof 200° C. for a period of 20 minutes.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a thermally-formable and crosslinkable precursorof a thermally-conductive material comprising a) one or morecrosslinkable polymers where the melt flow index of the polymer ormixture of polymers, respectively (measured at 190° C. according to ASTMD-1238 ) is 10-100 g/10 min and b) one or more thermally-conductivefillers in an amount of at least 60 wt. % of the total weight of theprecursor. Thermal conductivity can be measured using the methodspecified below. Materials having a thermal conductivity of at least0.30 W/m° C., preferably at least 0.35 W/m° C. and more preferably 0.40W/m° C. are termed above and below as thermally-conductive. Above andbelow, the term “thermally-formable” means capable of being altered inshape under the influence of heat.

The precursor comprises one or more polymers where the polymer ormixture of polymers, respectively, has a melt flow index of 10-100 g/10min, preferably 15-95 g/10 min and especially preferably 15-90 g/10 min(as measured according to ASTM D-1238 at 190° C.) and is capable ofbeing crosslinked. Polymers which meet these requirements are preferablyselected from the group comprising polyolefins, polyurethanes,silicones, polyvinyl chloride, polyvinyl ethers, polyvinyl acetate andpolystyrenes. The group of polyolefins and polyurethanes is preferred,however, as these materials tend to provide thermal stability andflexibility required by adhesive tape applications.

The precursor preferably comprises 1-5, more preferably 1-3 andespecially preferably 1-2 polymers where the polymer or mixture ofpolymers, respectively, has a melt flow index of 10-100 g/10 min,preferably 15-95 g/10 min and more preferably 20-90 g/10 min. If thepolymer comprises polymers having a melt flow index of between 10-100g/10 min and polymers having a melt flow index outside of this range,the amounts of such polymers are preferably selected so that the meltflow index of the mixture is between 10-100 g/10 min.

The polymer of the precursor may also optionally comprise othernon-crosslinkable polymers in an amount of up to 30 wt. %, and morepreferably up to 15 wt. %, based on the total weight of the precursor.

Polyolefinic polymers include homopolymers and copolymers of ethylene,propylene and butene and also include copolymers of olefins with othervinyl-group containing monomers such as (meth)acrylates, alpha-olefinssuch as 1-hexene and 1-octene, vinyl acetate and vinyl aromatics such asstyrene.

Polyurethane polymers useful in the present invention include bothsaturated and unsatruated polyurethane polymers, prepared by knownmethods from diosocynates and diols, respectively.

The precursor comprises one or more polymers, where the polymer ormixture of polymers, respectively, has a melt flow index (MFI) of 10-100g/10 min as measured at 190° C. by ASTM D-1238. Polymers or mixtures ofpolymers, respectively, that have a melt flow index of less than 10 g/10min tend to be difficult to extrude because of their high molecularweight. Polymers with a melt flow index of less than 10 g/10 min have arelatively high melt viscosity and the viscosity of the precursor tendsto increase as high amounts of fillers are added. Thus polymers with amelt flow index (MFI) of less than 10 g/10 min are not suitable for usein the present invention.

Polymers with a melt flow index (MFI) of greater than 100 g/10 min arealso unsuitable for use in the present invention as they have arelatively low molecular weight. These polymers would need to have theireffective molecular weight increased greatly after thermal forming intoa film, for example, to provide the thermally stability andhigh-temperature performance required of the thermally-conductiveadhesive tape end-product. Crosslinking capability can be introducedinto the precursor so as to provide a route to effectively increase themolecular weight after extrusion and to improve thermal stability ofpolymers. Crosslinking is more conveniently and frequently used,however, to increase the molecular weight of polymers which already havea substantial molecular weight. Thus polymers with a melt flow index ofgreater than 100 g/10 min are also unsuitable for use in the precursorof the present invention.

The polymer or polymers, respectively, selected for use in the precursormust also be capable of being crosslinked. Crosslinking of the polymermay be effected by several methods, including radiation crosslinking byultraviolet (UV) or gamma(γ)-radiation, by particle beam radiation(e-beam radiation), thermal cross-linking and crosslinking viamoisture-curing. Crosslinking by UV-radiation and e-beam radiation hasthe disadvantage that it often results in gradient curing effectsthrough the thickness of the thermally-formed and crosslinked material.Thermal crosslinking may be employed, but has the disadvantage thatthermal curing may already take place during the thermal forming byextrusion, for example, as relatively high temperatures required forextrusion are often similar to temperatures required for thermalcrosslinking. Crosslinking of the precursor via moisture-curing orγ-radiation is therefore preferred.

A suitable radiation source for γ-radiation is ⁶⁰Co which has twoγ-transitions of 1.17 and 1.33 MeV, respectively. Irradiation time incommercially operated ⁶⁰Co irradiation source facilities can be rented,for example, at BGS Beta-Gamma-Service GmbH & Co. Kg, Wiehl, Germany.The set-up and geometry of this ⁶⁰Co irradiation source facility isschematically described, for example, in the BGS brochure“Strahlenvernetzung von Kunststoffen” available from BGSBeta-Gamma-Service.

γ-irradiation which is useful in the present invention preferably has anenergy of between 50 keV-25 MeV and more preferably of 500 keV-10 MeV.An especially preferred γ-irradiation source is ⁶⁰Co.

It was found by the present inventors that the irradiation time with theγ-irradiation is selected so that the resulting irradiation dosagepreferably is at least 50, more preferably at least 80 and especiallypreferably at least 100 kGy.

Polymers which are capable of undergoing crosslinking viamoisture-curing include, for example, polyurethanes and polymers bearingreactive silane groups.

Polyurethanes undergo moisture curing by reaction of isocyanate groupswith ambient moisture, while polymers bearing reactive silane groupsundergo moisture curing by silanol condensation reactions orhydrosilation in the presence of platinum-based catalysts. The reactivesilane groups can be introduced into suitable polymers via a graftingreaction.

Polymers for use in the thermally-formable and crosslinkable precursorof the present invention which are suitable for silane grafting includepolyolefin polymers. The polyolefin polymer selected for silane graftingpreferably comprises at least 30%, more preferably at least 35% andespecially preferably at least 40% by weight of polyethylene units. Thepresence of at least 30% by weight of polyethylene units provides aneffective number of grafting sites on the polymer backbone, which inturn provides an effective amount of crosslinking after thermal formingof the precursor into a specific shape, such as a film, for example.

Polyethylene polymers suitable for silane grafting include polyethylenehomopolymers of a wide variety of densities, as well as polyethylenecopolymers with (meth)acrylate monomers, vinyl acetate and alpha-olefinssuch as propylene, butene, pentene, 1-hexene and 1-octene.

A first group of specifically preferred polyolefins suitable for silanegrafting comprises ultra low density polyethylenes (also known as verylow density polyethylenes) having a density of less than 0.90 g/cm³.Such materials are commercially available, for example, as

-   ENGAGE™ 8400 (ethylene-co-octene having a density of 0.870 g/cm³    according to ASTM D-792, a melt flow index (MFI) according to ASTM    D-1238 (190° C., 2.16 kg) of 30 g/10 min and a melting peak as    determined by DSC at a rate of 10° C./min of 60° C.);-   ENGAGE™ 8411(ethylene-co-octene having a density of 0.880 g/cm³    according to ASTM D-792, a melt flow index (MFI) according to ASTM    D-1238 (190° C., 2.16 kg) of 18 g/10 min and a melting peak as    determined by DSC at a rate of 10° C./min of 72° C.);-   ENGAGE™ 8401(ethylene-co-octene having a density of 0.885 g/cm³    according to ASTM D-792, a melt flow index (MFI) according to ASTM    D-1238 (190° C., 2.16 kg) of 30 g/10 min and a melting peak as    determined by DSC at a rate of 10° C./min of 78° C.);-   ENGAGE™ 8130(ethylene-co-octene having a density of 0.864 g/cm³    according to ASTM D-792, a melt flow index (MFI) according to ASTM    D-1238 (190° C., 2.16 kg) of 13 g/10 min and a melting peak as    determined by DSC at a rate of 10° C./min of 50° C.) and-   ENGAGE™ polyolefins are available from Dow DuPont Elastomers    (Geneva, Switzerland). Especially preferred materials from this    group are ENGAGE™ 8400 and ENGAGE™ 8407.

The ultra low density polyethylene polymer is preferably selected sothat the melting peak of the polymer as measured by DSC at a rate of 10°C./min is less than 100° C., more preferably less than 90° C. andespecially preferably between 60° C. and 80° C.

A second group of preferred polyethylenes suitable for silane graftingand specifically preferred for use in the precursor of the presentinvention comprises copolymers of ethylene and (meth)acrylate monomers,available, for example, as

-   LOTRYL™ 35 BA 40, a copolymer of ethylene and butyl acrylate    (co-E-BA) in a ratio of 65 parts ethylene to 35 parts butyl acrylate    having a density of 0.930 g/cm³ and a melt flow index (MFI)    according to ASTM D-1238 (190° C.,.2.16 kg) of 40 g/10 min);-   LOTRYL™ 17 BA 07, a copolymer of ethylene and butyl acrylate    (co-E-BA) in a ratio of 83 parts ethylene to 17 parts butyl acrylate    having a density of 0.930 g/cm³ and a melt flow index (MFI)    according to ASTM D-1238 (190° C., 2.16 kg) of 6.5-8 g/10 min);-   LOTRYL™ 28 BA 175, a copolymer of ethylene and butyl acrylate    (co-E-BA) in a ratio of 72 parts ethylene to 28 parts butyl acrylate    having a density of 0.930 g/cm³ and a melt flow index (MFI)    according to ASTM D-1238 (190° C., 2.16 kg) of 150-200 g/10 min);    and-   LOTRYL™ 28 MA 07, a copolymer of ethylene and methyl acrylate    (co-E-MA) in a ratio of 72 parts ethylene to 28 parts methyl    acrylate having a density of 0.930 g/cm³ and a melt flow index (MFI)    according to ASTM D-1238 (190° C., 2.16 kg) of 6-8 g/10 min).-   LOTRYL™ copolymers are available from ATOFINA (Duesseldorf,    Germany).

Other suitable ethylene-co-(meth)acrylate polymers suitable for silanegrarting and suitable for use in the precursor of the present inventionare those which also comprise polymerized units of maleic anhydride.These materials are commercially available from Elf Atochem (Puteaux,France) as

-   LOTADER™ 6200, a terpolymer of ethylene, acrylic ester and maleic    anhydride, having a comonomer content of 9%, a melt flow index    according to ASTM D-1238 (190° C., 16 kg) of 40 g/10 min and a    melting point by DSC of 102° C.;-   LOTADER™ 8200, a terpolymer of ethylene, acrylic ester and maleic    anhydride, having a comonomer content of 9%, a melt flow index    according to ASTM D-1238 (190° C., 2.16 kg) of 200 g/10 min and a    melting point by DSC of 100° C.;-   LOTADER™ 5500, a terpolymer of ethylene, and acrylic ester and    maleic anhydride having a comonomer content of 22%, a melt flow    index according to ASTM D-1238 (190° C., 2.16 kg) of 20 g/10 min and    a melting point by DSC of 80° C.; and-   LOTADER™ 7500, a terpolymer of ethylene, and acrylic ester and    maleic anhydride having a comonomer content of 20%, a melt flow    index according to ASTM D-1238 (190° C., 2.16 kg) of 70 g/10 min and    a melting point by DSC of 76° C.

Preferably a single polymer having a melt flow index (190° C., 2 16 kg)of 10-100 g/10 min, such as LOTRYL™ 35 BA 40, is selected from the groupof ethylene-co-(meth)acrylate polymers.

The melting point of polymers selected from the group ofethylene-co-(meth)acrylate polymers is preferably less than 100° C. andmore preferably less than 80° C.

The crosslinkable polymers or polymers for use in the precursor of thethermally-conductive material of the present invention are preferablyselected to give flexibility to the thermally-formed and crosslinkedmaterial. Flexibility is important to allow conformability of theadhesive tape to rough surfaces and to allow it to fill uneven orirregular bonding spaces or gaps. Polymers having a glass transitiontemperature, Tg, in the range of between −60° C. and −10° C. in theuncrosslinked state often exhibit such desirable characteristics.

The polymer is also preferably selected to impart strength to thepolymeric film backing so that the resulting thermally-conductiveadhesive tape can be handled, converted and applied to the substrateswithout stretching or breaking.

The polymer is also selected to be thermally stable. Preferred polymersfor use in the precursor are those that are thermally stable (no weightloss) as measured by thermogravimetric analysis (TGA) at 350° C. In air,according to DIN IEC 60811-4-1.

Reactive silane groups capable of undergoing moisture-crosslinking maybe incorporated into polymers in specific locations, such as on one orboth ends, for example. Reactive silane groups may also be incorporateddirectly into copolymer backbones in a random fashion bycopolymerization of unsaturated silane monomers. Reactivesilane-functionality may also be also introduced into polymers byfree-radically induced grafting techniques.

Such grafting techniques for introducing reactive silane functionalityonto polymers which are preferred in the present invention are describedin U.S. Pat. No. 3,646,155, U.S. Pat. No. 4,291,136, British PatentSpecification GB 1 357 549, British Patent Specification GB 1 406 680,British Patent Specification GB 1 450 934 and German Patent DE 44 02943, for example.

Silanes which can be employed in the grafting reaction have the generalformula (I) R R′SiY₂. R represents a monovalent olefinically unsaturatedradical attached to silicon through a silicon to carbon bond. Examplesof such radicals are allyl, vinyl, butenyl and cyclohexenyl, where vinylis preferred. Y represents a hydrolyzable organic radical, such as, forexample, an alkoxy radical such as methoxy, ethoxy and butoxy radicals.Y may also be an acyloxy radical such as formyloxy, acetoxy orpropionoxy, for example. Y may also be an oximo radical or a substitutedamino radical. The Y substituents in any given silane molecule may bethe same or different. R′ represents a monovalent hydrocarbon radicalfree of aliphatic unsaturation, such as, for example, methyl, ethyl,propyl, tetradecyl, octadecyl, phenyl, benzyl or tolyl. R′ may also be aY radical. Preferably the silane will have the formula RSiY₃, where R isvinyl, the most preferred silanes being vinyl triethoxysilane, vinyltrimethoxy silane and combinations thereof. The silane compoundsdescribed are meant to illustrate the invention without limiting it.

The amount of one or more silane compounds employed in the graftingreaction will depend on the reaction conditions and the type of polymeremployed. The present inventors have found that to advantageously graftand crosslink polymers having a melt flow index of 10-100 g/10 min, theamount of one or more silane compounds employed in the grafting reactionis preferably selected to be at least 2 parts per 100 parts polymer,more preferably at least 3 parts per 100 parts polymer and mostpreferably 3-5 parts silane per 100 parts polymer. If less than 2 partsis employed, then insufficient number of silane molecules are introducedonto the polymer backbone, an insufficient amount of crosslinking takesplace and the thermal stability of the resulting thermally-conductivematerial is too low.

Free-radical initiators suitable for promoting the grafting reaction arethose capable of generating free-radical sites on the base polymer to begrafted and which have a half-life at the reaction temperature ofpreferably less than about 6 minutes, and more preferably less than 1minute. Preferred free-radical initiators include organic peroxides andperesters such as, for example, benzoyl peroxide; dichlorobenzoylperoxide; dicumyl peroxide; di-t-butyl peroxide;1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane;2,5-dimethyl-2,5-di(peroxybenzoate)hexylene-3;1,3-bis(t-butylperoxyisopropyl) benzene; lauroyl peroxide; t-butylperacetate; 2,5-dimethyl-2,5(t-buytl peroxy)hexylene-3; t-butylperbenzoate, and azo compounds, for example, azobisisobutyronitrile anddimethyl azodi-isobutyrate. The selection of a particular free-radicalinitiator depends on the temperature at which the silane graftingreaction is to be performed. Dicumylperoxide is most preferred as afree-radical initiator.

The free-radical initiator or initiators can preferably be employed inthe amount of 0.10-0.30 parts per 100 parts polymer, more preferably at0.15-0.25 parts per 100 parts polymer and most preferably at about 0.175parts per 100 parts polymer. The present inventors have found thatpolymers or mixtures of polymers having a melt flow index (MFI) of10-100 g/10 min can be grafted and crosslinked advantageously byemploying the free-radical initiator in an amount of at least 0.1 partsper 100 parts polymer. If less than 0.1 parts is employed, theninsufficient grafting takes place, resulting in thermally-conductivematerials which have insufficient crosslinking. The silane graftingreaction is preferably performed at a temperature of between 120° C. and220° C., more preferably between 140° C. and 200° C. and most preferablybetween 160° C. and 180° C.

Moisture-curing of silane-grafted polymers is known to be promoted andaccelerated in the presence of silanol condensation catalysts. Thoughcatalysts accelerate the moisture-curing, it is not required that theybe employed in the precursor of the present invention. Known silanolcondensation catalysts include metal carboxylates such as dibutyltindilaurate, stannous ethyl hexanoate, stannous acetate, stannous octoate,and zinc octoate, for example. Organic metal compounds such as titaniumesters and chelates are also effective catalysts as are organic basessuch as hexyl amine. Acids such as mineral acids and fatty acids arealso known as silanol condensation catalysts. When a catalyst isemployed, organic tin compounds such as dibutyltin dilaurate, tin (II)ethyl hexanoate, dibutyltin diacetate and dibutyltin dioctoate arepreferred.

The thermally-formable and cross-linkable precursor of the presentinvention comprises, in addition to the crosslinkable polymer, one ormore thermally-conductive fillers. Any organic, inorganic or ceramicfiller that effectively enhances thermal conductivity of a polymericfilm may be employed as a filler in the precursor of the presentinvention.

Thermally-conductive fillers are defined above and below as those havinga thermal conductivity, λ, at 20° C., of at least 5.0 W/mK. Preferredthermally-conductive fillers are those which are electrically-insulatingas well as thermally conductive.

The group of preferred thermally-conductive fillers includes but is notlimited to alumina, aluminum oxide Al₂O₃, aluminum trihydroxide,magnesium hydroxide, beryllium oxide, magnesium oxide, zinc oxide, boronnitride, aluminum nitride and silicon carbide.

Most preferred thermally-conductive fillers are aluminum trihydroxideand magnesium hydroxide. An especially preferred thermally-conductivefiller is magnesium hydroxide. More preferred is magnesium hydroxidethat has been pre-treated with a vinyl silane. Such silane-treatedmagnesium hydroxide is commercially available, for example, as MAGNIFINH5A and MAGNIFIN H5MV from Alusuisse Martinswerk GmbH (Bergheim,Germany).

Suitable thermally-conductive filler or fillers for use in the precursorof the present invention are also preferred in particulate form.“Particulate filler”, as defined above and below, is a finely dividedsolid filler having an average particle size of less than about 75 μm.Preferred size of particulate fillers for use in the precursor of thepresent invention are those with an average particle size of less that20 μm. Most preferably is an average particle size of between 2 μm and 5μm.

The thermally-conductive filler or fillers are present in the precursorof the present invention in an amount of at least 60% by weight of thetotal weight of the precursor. At levels of below 60 wt. %, thethermally formed and crosslinked material shows insufficient thermalconductivity. The thermally-formable precursor is preferably filled withthe maximum tolerable amount of filler to promote maximum thermalconductivity of the thermally-formed and thermally-conductivecrosslinked material. The maximum amount of filler has been attainedwhen the melt viscosity of the precursor becomes such that it cannot bethermally formed or extruded, for example, or when the polymer of theprecursor is no longer capable of holding the filler. The filler ispreferably present in the precursor of the present invention in theamount of at least 62.5 wt. % and more preferably 65 wt. % based on thetotal weight of the precursor.

The thermally-conductive filler or fillers of the precursor of thepresent invention can be chemically treated, for example, by treatmentwith silanes or stearic acid, to promote interaction with the polymerand to allow maximum filler loading in the precursor. Use of fillers inthe amount of less that 60% by weight tends to produce insufficientthermal conductivity. Use of filler in very high amounts, such asamounts of over 75% by weight, for example, tends to raise the meltviscosity of the precursor and render it extremely difficult to extrude.Very high loading of filler also tends to produce thermally-conductivematerials that have poor cohesive strength.

The precursor of the present invention may also comprise additionalfillers and additives to the extent that the thermal conductivity andother physical properties of the thermally-formed and crosslinkedmaterial are not overly compromised. Low amounts of a solvents or inertdiluents may be employed during compounding and extrusion to lower theviscosity of the precursor and improve mixing of the polymer and thefiller, for example. Certain reactive diluents such as low molecularweight polymers comprising epoxide or acid groups may also be employed.Conventional wetting agents, anti-foaming agents, pigments,flame-retardants and antioxidants may be added to the precursordepending upon the requirements of the particular applicationenvisioned. Preferably, the sum of the amounts of additional fillers andadditives is an amount of less than 20 wt. % based on the total weightof the precursor. More preferably, the sum of the amounts of auxiliaryfillers and additives is an amount of less than 10 wt. % based on thetotal weight of the precursor.

A particularly preferred additive for the precursor of the presentinvention is a low molecular weight organic extender. A low molecularweight organic extender is defined above and below as a monomeric orpolymeric organic substance having a weight average molecular weight,M_(w), of less than about 20,000. The organic extender may be selectedfrom the group of materials commonly known as extender oils such asmineral oil and plasticizers such as dioctyl phthalate (DOP). The lowmolecular weight organic extender is present in the amount of at least 5wt. % based on the total weight of the precursor. Preferably theextender is present in the amount of between 10 wt. % and 20 wt. %.

The thermally-conductive filler or fillers are mixed with thecrosslinkable polymer before the precursor is thermally-formed into itsfinal shape. In general, compounding of the crosslinkable polymer withfiller can most easily be accomplished by adding the filler or fillersto the polymer melt in a heated mixing device such as an internal mixer(a Branbury™-type mixer, for example) or an extruder.

The thermally-formable precursor of the present invention is tack freeat 23° C. and cannot be characterized as having pressure-sensitiveadhesive (PSA) properties. The term “tack free” means qualitatively thatthe crosslinked precursor is not sticky to the touch. The term “tackfree” means quantitatively that the crosslinked precursor has a 90° peeladhesion value from stainless steel of less than 0.5 N/cm as measuredaccording to AFERA (Association des Fabricants Europeens de RubansAuto-Adhesivs) 4001.

The invention also provides a method of manufacturing the precursor ofthe present invention comprising the steps of a) providing one or morecrosslinkable polymers where the polymer or mixture of polymers,respectively, has a melt flow index of 10-100 g/10 min (as measuredaccording to ASTM D-1238 at 190° C.) and b) compounding the polymer orpolymers, respectively, with a thermally-conductive filler in an amountof at least 60 wt. % of the total weight of the precursor in a heatedmixing device. The invention furthermore provides a preferred method ofmanufacturing the precursor that specifically relates to embodimentswhere silane-grafting and moisture curing of the precursor are employed.This method of manufacturing the precursor comprises the steps of a)providing one or more polymers where the polymer or mixture of polymers,respectively, has a melt flow-index of 10-100 g/10 min (as measuredaccording to ASTM D-1238 at 190° C.) b) reacting the polymer or polymerswith a vinyl silane having the formula RR′SiY₂ (I), wherein R is amonovalently olefinically unsaturated radical, R′ is a monovalentradical free of aliphatic unsaturation and Y is a hydolyzable organicradical, and a free-radical initiator in a heated mixing device toproduce a moisture-curable polymer and c) compounding themoisture-curable polymer with one or more thermally-conductive fillersin an amount of at least 60 wt. % of the total weight of the precursorin a heated mixing device. The crosslinkable polymer preferably has anethylene unit content of at least 30% by weight with respect to thetotal weight of the crosslinkable polymer and the vinyl silanepreferably has the formula RR′SiY₂. Preferred embodiments of R, R′ and Yare described above.

The free-radical induced grafting reaction of the silane onto thepolymer is preferably conducted at temperatures of 120° C. to 220° C. ina heated mixing device such as an internal mixer (a Branbury™-typemixer, for example) or an extruder. If the grafting reaction temperatureis too low, the viscosity of the polymer is too high to achieve goodmixing with the vinyl silane and the free-radical initiator and theinitiator may fail to decompose thermally as required. If the graftingreaction temperature is too high then disadvantageous side reactions arepromoted.

Residence times for the grafting mixture in the heated mixing device ofless than 10 minutes are preferred, with a residence time of less than 5minutes being most preferred.

In embodiments where silane grafting is employed to render the polymercrosslinkable, the thermally-conductive filler or fillers are preferablycompounded with the polymer melt after the silane grafting reaction hasoccurred, but before the silane-grafted polymer is crosslinked.Preferably the thermally-conductive filler or fillers are added in asubsequent step after grafting, in the same extruder, so that a reducednumber of extruding and/or mixing steps is required.

The silanol condensation catalyst, when employed, can be mixed into thepolymer melt as the polymer is thermally-formed into its final shape byextrusion, for example, or may be applied in solution to the surface ofan extruded polymeric film, for example. The rate of the moisture-curingor crosslinking reaction may be accelerated with heat. Temperatures ofbetween 23° C. and 70° C. may be employed. Complete moisture curing a100-300 μm thick film of the precursor of the present invention requiresapproximately one day at a temperature of 23° C. and ambient relativehumidity. Curing may be effected in 10-15 minutes at temperatures of 70°C., however.

The invention also provides a method of manufacturing a shapedthermally-conductive material, comprising the steps of providing thethermally-formable precursor of the present invention, thermally formingthe precursor to a desired shape and then crosslinking the precursor.

The precursor can be shaped by any known method for forming polymericmaterials. Thermal forming of the precursor can comprise injectionmolding, for example, to form a three-dimensional body. Thermal formingof the precursor may also be performed by extrusion, for example, toform a sheet, ribbon or thin film. Thermal forming of the precursor intoa thick sheet may be conducted by calendaring techniques as well.

In general, thermal formability of the precursor is ensured bypreferably selecting the polymers of the precursor from the groupcomprising known thermoplastic polymeric materials capable of flowingwith reduced viscosity when heated. Thermoset polymeric materials, suchas epoxies, for example, are less preferred for use in the precursor ofthe present invention because of their tendency to undergo irreversiblechemical reaction under conditions commonly employed for processing theprecursor prior to the crosslinking step.

The invention also describes a thermally-conductive material obtainableby crosslinking the precursor of the present invention. The thermallyconductive material of the present invention is tack free at 23° C. andcannot be characterized as having pressure-sensitive adhesive (PSA)properties. The term “tack free” means qualitatively that thecrosslinked precursor is not sticky to the touch. The term “tack free”means quantitatively that the crosslinked precursor has a peel adhesionvalue from stainless steel of less than 0.5 N/cm as measured accordingto AFERA (Association des Fabricants Europeens de Rubans Auto-Adhesivs)4001.

The crosslinked thermally-conductive material of the present inventionis preferably crosslinked to the extent that it exhibits an elastictorque, S′, of 3-8 dNm, more preferably 4-7 dNm, as measured by ASTM D6294-9 after crosslinking at 200° C. for 20 minutes. Materials having anelastic torque, S′, of less than 3 dNm tend to have inferior thermalstability and poor high temperature performance under real useconditions. Materials having an elastic torque, S′, of greater than 8dNm tend to lack conformability and flexibility.

FIG. 1 shows the crosslinking behavior of the precursor of Examples 3and 4. Elastic torque, S′, of the precursor is shown as a function oftime for a period of 20 minutes while being held at a temperature of200° C.

The precursor of the present invention is preferably thermally-formedinto the shape of a film. Film lends itself easily to the formation ofinterfaces between relatively flat substrates and can easily be providedwith coating or other thin layers such as adhesives. The precursor isalso preferably formed into a film as this facilitates application ofcrosslinking techniques. Radiation curing can be employed effectively onfilms and, in embodiments where moisture-curing is preferred, thin filmsallow adequate contact with ambient moisture to effect crosslinking viamoisture curing within reasonable times and temperatures.

The thermally-conductive film suitable for use in thethermally-conductive adhesive tape of the present invention preferablyhas a thickness of 40-300 μm. Especially preferred is a film thicknessof 100-200 μm. If the film is too thick, then the thermal impedance istoo high. If the film is too thin, the breakdown voltage of theresulting tape tends to be too low, the backing is too thin to handleand it cannot contribute sufficiently to gap-filling properties requiredof the adhesive tape.

The thermally-conductive film suitable for use in thethermally-conductive adhesive tape of the present invention preferablyis flexible, conformable and elastic. Such qualitative characteristicsare reflected quantitatively in measurable film properties such as theE-modulus. Preferably the thermally-conductive film has an E-modulus of6-100 N/mm² and more preferably 20-50 N/mm².

The present invention also provides an adhesive tape comprising at leasta film backing bearing an adhesive layer on at least one major surfaceof the film backing, wherein the film backing is obtainable by extrudingthe precursor of the present invention into a film and crosslinking thefilm. Preferred classes of adhesive materials are those known to havegood thermal stability and resistance to aging, degradation andoxidation at high temperatures. Preferred adhesive polymer classesinclude acrylates, silicones and urethanes.

The adhesive layer applied to one or both major surfaces of the filmbacking may comprise a pressure-sensitive adhesive (PSA) which isinherently tacky at 23° C. or a heat-activatable adhesive that must beheated to develop tack.

Pressure-sensitive adhesive layers are preferred. Preferredpressure-sensitive adhesives (PSAs) are acrylate-basedpressure-sensitive adhesives. Acrylate-based pressure-sensitiveadhesives and methods of their preparation are described in Handbook ofPressure-Sensitive Adhesives (Ed. D. Satas, Third Edition, 1999).Acrylate-based PSAs comprise polymers or copolymers of acrylic and/ormethacrylic esters that are inherently soft and tacky polymers having alow glass transition temperature (T_(g)). Preferred (meth)acrylatemonomers for use in preparation of acrylic PSAs suitable for use in thepresent inventions comprise alkyl acrylate and methacrylate esters ofnon-tertiary alcohols comprising 4-17 carbon atoms where the homopolymerof the monomer has a T_(g) of less than about 0° C. Monomers suitablefor use in acrylic PSAs which can be employed in the present inventioninclude, for example, n-butyl acrylate, isobutyl acrylate, hexylacrylate, 2-ethyl-hexylacrylate, isooctyl acrylate, caprolactoneacrylate, isodecyl acrylate, tridecylacrylate, laurylmethacrylate,methoxy-polyethylenglycol-monomethacrylate, lauryl acrylate,tetrahydrofurfuryl acrylate, ethoxyethoxyethyl acrylate and ethoxylatednonylacrylate. Especially preferred are 2-ethyl-hexyl acrylate, isooctylacrylate and butyl acrylate. The low T_(g) (meth)acrylate monomer iscommonly present in preferred (meth)acrylate-based PSAs in an amount of50-100% by weight.

Optionally, one or more ethylenically unsaturated co-monomers may becopolymerzed with the (meth)acrylate ester monomer described above inamounts of between 0 and 50 weight % to improve cohesive strength of theadhesive and/or provide other desirable characteristics. One class ofuseful comonomers includes those having a homopolymer glass transitiontemperature (T_(g)) greater than the glass transition temperature of thehomopolymer of the (meth)acrylic esters listed in the precedingparagraph. Examples of suitable monomers falling within this classinclude acrylic acid, acrylamide, methacrylamide, substitutedacrylamides such as N,N-dimethyl acrylamide and N,N-diethylacrylamide,itaconic acid, methacrylic acid, vinyl acetate, N-vinyl pyrrolidone,isobornyl acrylate, cyanatoethyl acrylate, N-vinylcaprolactam, maleicanhydride, hydroxyalkyl-acrylates, N,N-dimethylaminoethyl(meth)acrylate,beta-carboxyethylmethacrylate, vinylidene chloride, styrene, vinyltoluene and alkyl vinyl ethers.

The pressure-sensitive adhesive layer may also be crosslinked to improveits performance, in particular shear strength, at elevated temperatures.Crosslinking of the adhesive layer may be provided by radiationpost-crosslinking, using e-beam or UV radiation, for example, or bythermal post-crosslinking using bis-amide type crosslinkers, forexample, or may be provided during the synthesis of the acrylic polymerby including multifunctional acrylates, such as hexanediol diacrylate,in the monomer mixture.

The adhesive layer may also comprise one or more thermally-conductivefillers. The filler type can be selected from the particulate fillersused in the above precursor. The thermally-conductive filler or fillers,when present, may be present in the adhesive layer in any amount whichis effective in enhancing the thermal-conductivity of the adhesive tapeof the present invention. The amount of filler or fillers in theadhesive layer should not adversely effect adhesive behavior so that theadhesive tape does not meet user requirements, however. If one or morefillers is present in the adhesive layer, an amount of less than 30 wt.% based on the total weight of the adhesive is preferred. Preferably,one or more fillers are employed in an amount of less than 15 wt. %based on the total weight of the adhesive. More preferably, the adhesivelayer comprises less than 5 wt. % of one or more fillers and isespecially preferably essentially free of thermally-conductive fillers.

The adhesive layer may also contain various additives commonly employedin adhesives, such as pigments, antioxidants, tackifiers, plasticizersand flame-retardants, for example. The amount of such additives ispreferably not more than 20 wt. % based on the total weight of theadhesive.

The thickness of the adhesive layer is preferred to be 10-50 μm and morepreferably 15-45 μm and especially preferably 15-25 μm.

Specifically preferred is an adhesive layer having a thickness of from15 μm-25 μm and where no thermally-conductive filler is present in theadhesive.

The adhesive layer may be applied to one or both major surfaces of thecrosslinked and filled polymeric film backing and may be applied by anyof the commonly known methods for generating thin adhesive films onsubstrates, including hot-melt extrusion coating of 100% solidsadhesives and application of adhesive from solvent, suspension oremulsion (followed by drying) using common techniques such asknife-coating, spraying, gravure-coating and screen-printing. Anadhesive layer may also be prepared separately as a thin layer ofadhesive supported on a release liner (a transfer tape) and thentransferred by lamination to one or both major surfaces of thethermally-conductive crosslinked film backing according to the presentinvention.

The adhesive may also be applied to one or both surfaces of thecrosslinked thermally-conductive film backing in a manner so that theadhesive only partially covers the major surface(s) to which it has beenapplied. This can be accomplished by screen-printing, for example, or bytransfer of a segmented adhesive layer formed separately on a releaseliner. Adhesive layers applied in a manner so as to give only partialcoverage of the polymeric film backing may be preferred in instanceswhere it is critical to prevent air-entrapment at the interface betweenthe adhesive and the substrate to which it is bonded, such as a PCBs ora heat sink, for example. Presence of air bubbles at theadhesive-substrate interface is known to reduce adhesive contact andreduce the ability of the adhesive to transmit thermal energy. Thepresence of small adhesive-free areas, in particular when using apressure-sensitive adhesive layer, can facilitate removal of air bubblesfrom the bonding interface. In cases where partial coating of thethermally-conductive film backing with adhesive is employed, the area ofone or both major surfaces of the thermally-conductive film backing ispreferably covered with adhesive to the extent of at least 90%, morepreferably 95%.

Other techniques may also be used to prevent entrapment of air bubblesat the bonding interface. One such preferred technique requiresintroduction of a three-dimensional character to the surface of theadhesive layer. This can be accomplished by contacting the adhesivesurface with a microstructured or roughened release liner that transfersits three-dimensional character to the surface of the adhesive layer.Thermally-conductive adhesive tapes bearing at least one adhesive layerwith embossments, channels or grooves are described in U.S. Pat. No.5,213,868 (Chomerics).

The adhesive tape of the present invention, including thethermally-conductive film backing having one or more adhesive layers,preferably has a thickness of less than 300 μm. More preferably, thethickness of the adhesive tape is between 100 μm and 275 μm. If thethickness of the adhesive tape is greater than 300 μm, its thermalimpedance tends to be too high. If the thickness of the adhesive tape isless than 100 μm, then the dielectric strength of the adhesive tape andits conformability to substrates is insufficient.

Adhesive tapes having a dielectric strength of at least 55 kV/mm arepreferred. Most preferred are adhesive tapes having a dielectricstrength of at least 60 kV/mm. The dielectric strength of the adhesivetape is an important property for end use. Dielectric strength can beused to quantitatively reflect the ability of a material to resist thepassage of electrical current and is measured according to DIN EN60243-1. Adhesive tapes having a dielectric strength of less than 55kV/mm exhibit insufficient electrical resistance and have a lowbreakdown voltage, i.e. that voltage at which an electric discharge iscapable of passing completely through the adhesive tape.

The invention also provides a thermally conductive adhesive tape havingan effective thermal conductivity of at least 0.4 W/m-K measuredaccording to ASTM D 5470-95. Adhesive tapes which exhibit an effectivethermal conductivity

of less than 0.4 W/m-K cannot be relied upon to transmit thermal energywith the efficiency and speed required by the use conditions in theelectronics industry.

The invention also provides a thermally-conductive adhesive tape havinga thermal Impedance of less than 6.0° C.-cm²/W as measured according toASTM D 5470-95.

Preferred are adhesive tapes which have a dielectric strength of greaterthat 55 kV/mm, an effective thermal conductivity of at least 0.4 W/m-Kand a thermal impedance of less than 6.0° C.-cm²/W.

The invention also refers to the use of the thermally-conductivecrosslinked films for forming a thermally-conductive interface betweentwo substrates, such as the surface of a heat-generating body and aheat-absorbing body, for example. The thermally-conducive crosslinkedfilm which is obtainable by forming the precursor into the shape of afilm with subsequent crosslinking, preferably comprises one or moreadhesive layers to form an adhesive tape.

The invention also refers to a specific use, where the heat-generatingbody Is a printed circuit board and the heat-absorbing body is a heatsink.

The invention also refers to an assembly comprising thethermally-conductive film of the present invention in a bondingrelationship between two substrates.

MATERIALS USED IN THE EXAMPLES AND COMPARATIVE EXAMPLES

A. Crosslinkable Polymers

ENGAGE 8400—ethylene-co-octene having a density of 0.870 g/cm³,available from Dow Dupont Elastomers (Geneva, Switzerland). Melt flowindex (MFI) according to ASTM D-1238 (190° C. 2.16 kg) of 30 g/10 min.

LOTRYL EA 35 BA 40—ethylene and butyl acrylate (co-E-BA) in a ratio of65 parts ethylene to 35 parts butyl acrylate having a density, of 0.930g/cm³, available from ATOFINA (Duesseldorf, Germany). “Melt flow index(MFI) according to ASTM D-1238 (190° C., 2.16 kg) of 40 g/10 min.

LOTRYL 28 MA 07—ethylene and methyl acrylate (co-E-BA) in -a ratio of 72parts ethylene to 28 parts methyl acrylate having a density of 0.95g/cm³, available from ATOFINA (Duesseldorf, Germany). Melt flow index(MFI) according to ASTM D-1238 (190° C., 216 kg) of 7 g/10 min.

B. Vinyl Silane of the Formula (1)

Vinyl-trimethoxy silane, available as DYNASYLAN VTMO from Degussa AG(Hanau, Germany).

C. Free-Radical Initiator

Dicumylperoxide or 2,2-bis-phenylpropyl peroxide, available as LuperoxDCSC (active oxygen level of 5.80-5.92 wt. %) from Atofina DeutschlandGmbH (Gunzburg, Germany).

D. Catalysts for Moisture-Curing (Optional)

Tin (II) ethyl hexanoate, available from Johnson Mathey GmbH (Karisruhe,Germany)

E. Thermally-Conductive Filler

MAGNIFIN H5A, Mg(OH)₂ powder, vinyl silane-treated, available fromAlusuisse Martinswerk GmbH (Bergheim, Germany). Specific Surface (BET)of 4.0-6.0 m²/g. Particle Size: d₅₀ 1.25-1.65 μm.

Test Methods:

Breakdown Voltage, kV

The breakdown voltage for the adhesive tapes was measured according toDIN (Deutsche Industrie Norm) EN 60243-1. The results were recorded inkV.

Dielectric Strength, kV/mm

The breakdown voltage of adhesive tapes was measured according to DIN EN60243-1. The results were normalized to account for the thickness of theadhesive tape measured.

Volume Resistivity, ohm-cm

The Volume Resistivity for adhesive tapes was measured according to DIN(Deutsche Industrie Norm) IEC 93. Results were recorded in ohm-cm.

Effective Thermal Conductivity and Thermal Impedance

Effective thermal conductivity and thermal impedance of the adhesivetapes were measured according to ASTM D 5470-95 (Thermal TransmissionProperties of Thin Thermally Conductive Solid Electrical InsulationMaterials) with the following modifications:

a. Test Apparatus

The heat source employed in the test equipment was an insulated copperblock (25.4 mm×25.4mm) and 3 mm thick which was heated electrically witha constant power. The cooling unit was a copper block cooled by watersupplied from a constant temperature bath such that the temperature ismaintained uniformly within ±0.20 K. The heat source and cooling unittemperatures were measured independently with thin thermocouples placedexactly opposing one another on opposite sides of the test specimen. Thethermocouple tip for the heat source was placed within the center of thecopper plate. The thermocouple tip for the cooling unit was placed nearthe surface of the block.

b. Test Procedure

For both effective thermal conductivity and thermal impedance, anadhesive tape specimen having a size of 25.4 mm×25.4 mm was employed.

First, the thickness of the adhesive tape test specimen was measured at23° C. The test specimen was then centered between a heat source copperblock at a higher temperature, T₁, and a cooling unit copper block at alower temperature, T₂. A thermocouple was inserted into both copperblocks. A load of 1.75 kg was then applied on top of an insulator on topof the upper, heat source copper block, to hold the test assemblytogether and insure intimate contact between the adhesive tape testspecimen and the copper blocks.

Cooling water was circulated to the cooling unit and power was appliedto the heating element in the heat source. The temperature of both theheat source and the cooling unit was recorded at equilibrium.Equilibrium was attained when two successive sets of temperaturereadings taken at 15 min intervals had a difference of less than 0.2 K.Voltage, V (in volts), and current, I (in amps), at equilibrium werethen recorded.

Specimens of the same adhesive tape having a variety of thicknesses werethen evaluated in the same manner to give an equilibrium temperature fora range of thicknesses.

c. Calculations

Effective Thermal Conductivity, k, in Units of W/m K

-   1. Calculate the heat flow, Q, from the applied electrical power:    Q=V×I, where V is voltage in volts and I is current in amps-   2. Calculate the effective thermal conductivity, k:    k=(Q×s)/(A×ΔT), where-   Q=heat flow in Watts-   s=thickness of the adhesive tape in meters-   A=area of the adhesive tape in sq. m.-   ΔT=T₁−T₂-   Results are reported in units of W/mK.    Thermal Impedance, Z, in Units of ° C.-cm²/W

Thermal impedance, Z, is defined as the temperature gradient per unit ofheat flux, (Q/A), passing through the adhesive tapeZ=ΔT×(A/Q) where

-   ΔT=T₁−T₂-   A=area in sq. m.-   Q=heat flow in Watts-   R=thermal resistance in ° C./W and gives results in units of °    C.-cm²MW. R, thermal resistance, is defined as Z/A and is measured    in units of ° C./W.    E-Modulus

Determination of modulus of the adhesive tapes of the invention wasconducted according to DIN (Deutsche Industrie Norm) 53455.

Evaluating of Rheological Properties and Crosslinking Behavior of thePrecursor

Samples of the uncrosslinked precursor having a weight of 6.8-7.0 gramsfor filled polymers and 3.9-4.2 grams for unfilled polymers wereemployed for the test in the shape of a disc. The sample discs werecured by irradiation with γ-irradiation. In case of moisture-curing, thesample discs were placed in a rotorless oscillating shear rheometercommercially available as Rheometer Model MDR 2000 from AlphaTechnologies located in Akron, Ohio/USA. Moisture-curing is was theneffected by heating the disc samples of the precursor in the rheometerchamber. Rheological properties of the precursor were measured andrecorded as a function of time.

Curing by γ-irradiation was effected by applying a distinct dosage ofthe radiation from the ⁶⁰Co irradiation source to the disc samples ofthe precursor prior to placing the disc samples into the rheometerchamber.

This test was performed according to ASTM D 6204-99 Standard Test Methodfor Rubber—Measurement of Unvulcanized Rheological Properties Using aRotorless Shear Rheometer. A strain of ±2.8% and a frequency of 0.5 Hzwere employed.

The elastic torque, S′, of the test materials comprising uncrosslinkedprecursor and moisture-curing cataylst after 20 minutes at 200° C., orof the test material after receiving a certain dosage of γ-irrdadiationwas reported in units of dNm.

Hot-Set Test

A general description of this test is given in Deutsche Industrie NormDIN EN 60811-2-1 for Cable Insulation (Section 9Waerme-Dehnungspruefung). Samples of the 150 μm thick film to be testedwere die cut from the bulk film using a standard die (Normstab 2)according to DIN (Deutsche Industrie Norm) 47/472/Part 602.

The load employed was 10 N/cm. The test was conducted at 150° C. Theelongation was measured after the samples had been in the oven for aperiod of 15 minutes.

EXAMPLES Example 1

a. Silane Grafting of the Polymer

A free-radically induced grafting reaction was conducted by firstintroducing pellets of an ethylene-co-octene polymer having a density of0.870 g/cm³, melt flow index of 30 g/10 min (measured at 190° C.according to ASTM D-1238), available as ENGAGE 8400 from Dow DupontElastomers (Geneva, Switzerland) into a first extruder (single-screw)with a mixture of vinyl-trimethoxy silane (in the amount of 2.0 parts byweight silane per 100 parts polymer (or 0.66 wt. % based on the totalweight of the filled precursor) and dicumylperoxide (in the amount of0.085 parts by weight per 100 parts polymer or 0.03 wt. % based on thetotal weight of the filled precursor). The residence time of the polymerin the first extruder was about three minutes.

b. Compounding Grafted Polymer with Thermally-Conductive Filler andExtruding

The silane-grafted polymer thus formed was fed directly into a secondextruder (twin-screw) and compounded with silane-treated Mg(OH)₂ powderavailable as MAGNIFIN™ H5A from Alusuisse Martinswerk (Bergheim,Germany) using a gravimetric feeder. Magnesium hydroxide was employed inthe amount of two parts by weight for each part by weight ofsilane-grafted polymer, resulting in a precursor composition with about66 wt. % filler based on total weight of the precursor. The grafted andfilled precursor was then pelletized.

A catalyst comprising tin (II) ethyl hexanoate was combined with thepellets of precursor in a third extruder in the amount of 0.2 parts per100 parts polymer (or 0.07 wt. % based on the total weight of.theprecursor) and the precursor was extruded into a film having a thicknessof 200 μm. Specifically, a single screw extruder and a slot die wereemployed for this thermal forming operation. The residence time of theprecursor in the extruder was limited to less than 5 minutes so as toavoid extensive crosslinking during the film extrusion process. Chemicalcomposition of the film of Example 1 is summarized in Table 1.

c. Moisture Curing of the Thermally-Conductive Film Backing

The thermally-conductive film thus formed was allowed to stand at 23° C.and 50% relative humidity for 24 hours before testing to allow moisturecuring of the silane-grafted ethylene-co-octene polymer. Thethermally-conductive film thus prepared was then evaluated by theModulus, Tensile and Elongation Test. Characteristics of the crosslinkedfilm backing are summarized in Table 2.

d. Preparation of a Thermally-Conductive Adhesive Tape

The thermally-conductive film prepared by the method described above wasfirst corona-treated on both major surfaces to improved anchorage of theadhesive layers.

A 20 μm thick layer of pressure-sensitive adhesive (a transfer tape) wasthen prepared by coating a solvent-based acrylic pressure-sensitiveadhesive comprising a copolymer of isooctyl acrylate and acrylic acid ina weight ratio of 96/4 onto a siliconized paper liner and dried at 110°C.

A thermally-conductive adhesive tape was then prepared by laminating thetransfer tape thus prepared to each side of the corona-treatedthermally-conductive backing.

The adhesive tape was then tested according to the methods set forthabove. Electrical and thermal properties of the tape of Example 1 aresummarized in Table 3.

Example 2

Example 2 was prepared in a manner similar Example 1 with the exceptionthat the amount of catalyst for promoting the moisture curing of thesilane-grafted polyethylene film was raised to 0.13 wt. % based on thetotal weight of the precursor. Tests were conducted on the extrudedthermally-conductive film backing and the adhesive tape as in Example 1.

Comparative Example 1

Comparative Example 1 was prepared in a manner similar to Example 1 withthe exception that the magnesium hydroxide filler was present in theamount of 55 wt. % based on the total weight of the precursor. Testresults of the adhesive tape shown in Table 3 show an effective thermalconductivity of 0.390 W/m-K. TABLE 1 Grafting agents Silane, Peroxide,Filler Polymer wt. % wt. % Mg(OH)₂, Catalyst, Ex. Type, Tradename MFIWt. % (pph*) (pph*) wt. % wt. % 1 eth-co- Engage 8400 30 33.08 0.66 0.0366.16 0.07 octene (2.0) (0.085) 2 eth-co- Engage 8400 30 33.06 0.66 0.0366.12 0.13 octene (2.0) (0.085) C1 eth-co- Engage 8400 30 44.00 1.960.08 53.76 0.20 octene (2.0) (0.085)wt. % = weight percent based on the total weight of the precursor*pph = parts per 100 parts polymer)

TABLE 2 Mod. Tens. max Tens. break Ex. (N/mm²) E, max (%) E, break (%)(N/mm²) (N/mm²) 1 1.3 66 84 5.3 4.5 2 1.3 70 80 5.1 4.4 C1 0.8 71 87 5.23.9

TABLE 3 Effective thermal Thermal Breakdown Dielectric Volumeconductivity impedance voltage strength resistivity Ex. W/m-K ° C. cm²/WkV kV/mm ohm-cm 1 0.454 4.96 15.1 — 2.9 × 10¹⁵ 2 0.435 4.95 13.0 63 3.1× 10¹⁵ C1 0.390 6.22 15.7 — 4.7 × 10¹⁵

Example 3

A copolymer of ethylene and butyl acrylate (ethylene-co-butyl acrylate)was obtained in pellet form as LOTRYL 35BA40 (ATOFINA, Duesseldorf,Germany). The polymer has a melt flow index of 40 g/10 min as measuredat 190° C. according to ASTM D-1238.

Dicumylperoxide (0.175 parts per 100 parts polymer or 0.057 wt. % basedon the total weight of the precursor) and vinyl-trimethoxy silane (4.7parts per 100 parts polymer or 1.542 wt. % based on the total weight ofthe precursor) were mixed together, then added to the pellets and mixed.The mixture then was fed into a single-screw extruder having thermalzones increasing in temperature from 160° C. to 220° C., where graftingof the silane onto the polymer backbone occurred, promoted by thermaldecomposition of the peroxide.

The silane-grafted polymer (100 parts by weight) was then compoundedwith the magnesium hydroxide (200 parts by weight) in a twin-screwextruder at 170° C., in a manner similar to that of Examples 1-2. Thegrafted and filled polymer was then pelletized.

The pellets of precursor were then extruded to a white opaque polymericfilm having a thickness of 200 μm without the addition of catalyst.

The extruded thermally-conductive film backing was then crosslinked byspraying the extruded film with a catalyst solution comprising 5% tin(II) ethyl hexanoate by weight in heptane. The film was allowed to standat 23° C. for 4 hours and was then washed with distilled water and driedat 40° C. for 1 hour. The chemical composition of the film backing issummarized in Table 4. Measurements made on the cross-linked filmbacking include thickness and E-modulus, as well as elastic torque, S′.Properties of the thermally-conductive film backing are summarized inTable 5.

The thermally-conductive film was then corona treated on both sides. A20 μm thick layer of acrylate-based pressure-sensitive adhesive havingessentially the same composition and thickness as that employed inExamples 1-2 was then laminated to each side of the film, forming athermally-conductive pressure-sensitive adhesive tape having a totalthickness of approximately 250 μm.

The adhesive tape was then evaluated for its thermal propertiesincluding thermal conductivity and thermal impedance, as well as for itselectrical properties including breakdown voltage, dielectric strength,effective thermal conductivity, thermal impedance and volumeresistivity.

The properties of the adhesive tape are shown in Table 6.

Example 4

Example 4 was prepared in the manner of Example 3 with twoexceptions: 1) the amount of vinyl silane employed in the graftingreaction was reduced to 3.45 parts per 100 parts polymer (or 1.136 wt. %based on the total weight of the precursor) and 2) the thickness of theextruded film backing was increased to ca. 220 μm. A pressure-sensitiveadhesive tape was prepared from the crosslinked film backing in the samemanner as for Example 3. Properties of the crosslinked film backing andthe adhesive tape made from it are summarized in Tables 5 and 6 below.

Example 5

Example 5 was prepared in the same manner as Examples 3-4, with theexception that the amount of vinyl silane employed in the graftingreaction was reduced further to 2.00 parts per 100 parts polymer (or0.662 wt. % based on the total weight of the precursor). The filmbacking was extruded at a thickness of ca. 166 μm.

The resulting adhesive tape had a thickness of ca. 206 μm.

Comparative Examples 2-3

Comparative Examples 2 and 3 show the behavior of precursors andpolymeric film backings having less than 60 wt. % particulatethermally-conductive filler based on the total weight of the precursor.Comparative Examples 2 and 3 are similar to Example 4 in that therelative amounts of polymer and grafting agents (silane and peroxide)remained unchanged. Comparative Example 2 only has ca. 40 wt. %thermally-conductive filler and Comparative Example 3 comprises nofiller.

The grafted polymers were extruded and crosslinked according to the samemethods as the examples of the invention, and made into adhesive tapes.The reduced level of filler and the absence of filler, respectively, isclearly reflected in the thermal properties of the adhesive tapes assummarized in Table 6.

Comparative Example 4

Comparative Example 4 employed the same polymer and filler, in the samerelative amounts, as in Examples 3-5. No grafting reaction wasperformed, however. Thus the polymer could not be crosslinked by amoisture-curing reaction.

Comparative Examples 5 and 6

Two experiments were performed to show the effect of employing a polymerhaving a melt flow index of less than 20 in the precursor of thethermally-conductive material.

A copolymer of ethylene and methyl acrylate (ethylene-co-methylacrylate) was obtained in pellet form as LOTRYL 28MA07 (ATOFINA,Duesseldorf, Germany). The polymer has a melt flow index of 7 g/10 min(as measured at 190° C. according to ASTM D-1238).

Comparative Example 5 was prepared by a method essentially the same asthat of Example 5, with the exception that the much higher molecularweight LOTRYL 28MA07 was employed as the crosslinkable polymer. LOTRYL28MA07 has a melt flow index of 7 g/10 min. Amounts of grafting agentsand grafting conditions suitable for polymers having a melt flow indexof 30 g/10 min were found to be unsuitable for a chemically-similarpolymer having a melt flow index of 7 g/10 min. The resulting extrudedfilm was stiff and unsuitable for used as a backing for athermally-conductive adhesive tape.

Comparative Example 6 was prepared using a 50/50 wt/wt mixture of LOTRYL17BA07 and magnesium hydroxide filler. The amounts of grafting agentswere selected so that the amount of vinyl silane employed wassubstantially reduced to 1 part per 100 parts polymer (or 0.985 w.t %based on the total weight of the precursor) and the level of dicumylperoxide employed as a free-radical initiator was raised substantiallyto 0.5 parts per 100 parts polymer (or 0.493 wt. % 10 based on the totalweight of the precursor.) Attempts to perform the grafting reactionresulted in a mass that crosslinked to such an extent under extrusionconditions that it was not possible to obtain an extruded film. TABLE 4Grafting agents Polymer Silane, Peroxide, Filler Trade- wt. % wt. %Mg(OH)₂, Ex. Type, name MFI Wt. % (pph*) (pph*) wt. % 3 E-co- LOTRYL 4032.800 1.542 0.057 65.601 BA 35BA40 (4.70) (0.175) 4 E-co- LOTRYL 4032.935 1.136 0.058 65.871 BA 35BA40 (3.45) (0.175) 5 E-co- LOTRYL 4033.093 0.662 0.058 66.187 BA 35BA40 (2.00) (0.175) C2 E-co- LOTRYL 4058.954 2.034 0.103 38.909 BA 35BA40 (2.00) (0.175) C3 E-co- LOTRYL 4096.502 3.329 0.169 0 BA 35BA40 (3.45) (0.175) C4 E-co- LOTRYL 40 33.3330 0 66.667 BA 35BA40 C5 E-co- LOTRYL 7 33.093 0.662 0.058 66.187 MA28MA07 (2.00) (0.175) C6 E-co- LOTRYL 7 49.261 0.985 0.493 49.261 MA28MA07 (1.0) (0.5)wt. % = weight percent based on the total weight of the precursor*pph = parts per 100 parts polymer

TABLE 5 E-Modulus of Elastic torque, Elastic torque, crosslinked S′, ofS′, of crosslinked themally-conductuve uncrosslinked thermally- filmbacking precursor conductive Example (N/mm²) (dNm) material (dNm) 3 33 ±9 0.66 5.73 4 31 ± 2 0.73 4.90 5 20 ± 2 — — C2 12 ± 1 — — C3  6 ± 1 0.060.35 C4 50 ± 3 — — C5 17 ± 4 1.63 5.86— = not measured

TABLE 6 Therm. Ex- conduc- Thermal Breakdown Dielectric Volume am-tivity impedance voltage Stength Resistivity ple W/m-° C. ° C.-cm²/W kVkV/mm (Ohm-cm) 3 0.466 5.222 16.2 68.6 1.44 × 10¹³ 4 0.484 5.388 17.265.8 1.20 × 10¹³ 5 0.476 5.320 13.8 67.3 1.61 × 10¹³ C2 0.422 5.095 14.271.1 1.11 × 10¹³ C3 0.250 10.731 17.0 69.0 6.44 × 10¹² C4 0.558 4.84615.8 71.3 1.20 × 10¹³ C5 0.495 5.97 14.2 56.3 6.60 × 10¹²

Example 6

A 150 μm thick film comprising 100 parts by weight ethylene-butylacrylate (EBA)co polymer (available as Lotryl 35 BA40 from Atofina) and200 parts by weight magnesium hydroxide (available as Magnifin H5A fromMartinswerk, Bergheum, Germany) was extruded onto a cooled steel rollusing a twin screw extruder. The film thus prepared had a width of 30cm.

A 2-meter length of film was then cross-linked by exposure to gammaradiation from an encapsulated Cobalt 60 source having a maximumcapacity of 5 Mci and power of 75 kW. The radiation dosage received bythe film was 60 kilogray (kGy).

The cross-linked film was then tested for heat-resistance according tothe method described above as Hot-Set Test under TEST METHODS. This testgives an indication of the degree of cross-linking that has occurred dueto gamma radiation treatment, in that films with higher degree ofcross-linking give a lower elongation.

The test showed that the 150 μm thick film had an elongation of 30%after being exposed to 150° C. for 15 minutes under a load of 10 N/cm ascalled for by the test method. Results are summarized in Table 7.

The irradiated film was then used as a backing for preparation of athermally-conductive adhesive tape by the procedure described in Example1 (part d.).

Examples 7 and 8

Example 6 was repeated with the exception that the 150 μm thick film wasexposed to radiation to such an extent that the total dosage forExamples 7 and 8, respectively, was 120 kGy and 150 kGy. The films ofExamples 7 and 8 were tested according to the Hot-Set test and resultedin an elongation of 15% for Example 7 and an elongation of 16% forExample 8.

Comparative Examples 7-9

Un-radiated, non-cross-linked film (Comparative Example 7) was evaluatedby the Hot-Set Test as well as for elastic torque, S′, as describedunder Test Methods above.

Films having a low level of cross-linking caused by exposure to a lowerradiation dosages of only 30 kGy and 45 kGy, respectively, (ComparativeExamples 8 and 9) extended during the test and tore after a few seconds.TABLE 7 Radiation dosage Elongation Elastic torque, S′ Example (KGy) (%)(dN/m) C7 0 *  0.14 C8 30 * — C9 45 * — 6 60 30 3.5 7 120 15 8.4 8 15016 9.8* Sample tears after <5 seconds— Not measured

1-29. (canceled)
 30. A method of making an adhesive tape comprising: a)providing a crosslinkable polymer or a mixture of crosslinkablepolymers, wherein the melt flow index of the polymer or mixture ofpolymers is 10-100 g/10 min as measured at 190° C. and 2.16 kg accordingto ASTM D-1238; b) compounding the polymer(s) with one or morethermally-conductive fillers to provide a crosslinkable precursor of athermally-conductive material, wherein the precursor comprises at least60% by weight of the thermally conductive fillers; c) forming thecross-linkable precursor into the shape of a film backing; d)crosslinking the film backing so that the film backing has an elastictorque S′ of at least 3 dNm as measured according to ASTM D 6294-9; ande) providing an adhesive layer on at least one major surface of the filmbacking.
 31. The method of claim 30, wherein the crosslinkable polymersare selected from the group consisting of polyolefins and polyurethanes.32. The method of claim 30, wherein at least one crosslinkable polymeris a polyolefin having at least 30% by weight ethylene units, optionallywherein the polyolefin is a copolymer comprising ethylene and(meth)acrylate ester units.
 33. The method of claim 30, wherein at leastone of the crosslinkable polymers comprises one or more moisture-curablegroups, optionally wherein the moisture-curable groups comprise silanegroups.
 34. The method of claim 33, wherein providing the crosslinkablepolymer comprising one or more moisture-curable groups comprisesreacting a polymer with one or more vinyl silane compounds of theformula RR′SiY₂, a free-radical initiator, and, optionally, a catalystfor moisture-curing of the moisture-curable group; wherein R is amonovalently olefinically unsaturated radical, R′ is a monovalentradical free of aliphatic unsaturation, and Y is a hydrolyzable organicradical, optionally wherein the vinyl silane compound(s) are employed inan amount of at least 2 parts per 100 parts crosslinkable polymer orpolymers.
 35. The method of claim 34, wherein the free-radical initiatoris selected from the group consisting of organic peroxides and organicperesters, optionally wherein the free-radical initiator is employed inthe amount of at least 0.1 parts per 100 parts crosslinkable polymer orpolymers.
 36. The method of claim 30, wherein the thermally-conductivefiller is selected from the group consisting of alumina, aluminum oxide,aluminum trihydroxide and magnesium hydroxide.
 37. The method accordingto claim 30, wherein cross-linking the film comprises applyingγ-irradiation, optionally wherein the γ-irradiation has an energy ofbetween 50 keV-25 MeV, and optionally wherein the γ-irradiation dosageis at least 50 kGy.
 38. The method according to claim 30, whereincross-linking the film comprises moisture-curing.
 39. An adhesive tapemade according to the method of claim
 30. 40. An adhesive tapecomprising a film backing and an adhesive layer on at least one majorsurface of the film backing, wherein the film backing comprises acrosslinked, thermally-conductive material comprising a) one or morecrosslinked polymers, wherein the melt flow index of the polymer ormixture of polymers prior to crosslinking is 10-100 g/10 min as measuredat 190° C. and 2.16 kg according to ASTM D-1238; and b) at least 60% byweight of one or more thermally-conductive fillers, based on the totalweight of the thermally-conductive material; wherein the crosslinkedfilm backing has an elastic torque S′ of at least 3 dNm as measuredaccording to ASTM D 6294-9; optionally wherein the adhesive is apressure-sensitive adhesive.
 41. The adhesive tape of claim 40, whereinthe crosslinkable polymers are selected from the group consisting ofpolyolefins and polyurethanes.
 42. The adhesive tape of claim 40,wherein the crosslinkable polymer is a polyolefin having at least 30% byweight ethylene units, optionally wherein the polyolefin is a copolymercomprising ethylene and (meth)acrylate ester units.
 43. The adhesivetape of claim 40, wherein at least one of the crosslinkable polymerscomprises one or more moisture-curable groups, optionally wherein themoisture-curable groups comprise silane groups.
 44. The adhesive tape ofclaim 43, wherein the crosslinkable polymer comprising one or moremoisture-curable groups comprises the reaction product of a polymer withone or more vinyl silane compounds of the formula RR′SiY₂ wherein R is amonovalently olefinically unsaturated radical, R′ is a monovalentradical free of aliphatic unsaturation, and Y is a hydrolyzable organicradical, optionally wherein the vinyl silane compound(s) are employed inan amount of at least 2 parts per 100 parts crosslinkable polymer orpolymers.
 45. The adhesive tape of claim 40, wherein thethermally-conductive filler is selected from a group consisting ofalumina, aluminum oxide, aluminum trihydroxide and magnesium hydroxide.46. The adhesive tape of claim 40, wherein the tape has a dielectricstrength of at least 55 kV/mm as measured according to DIN EN 60243-1.47. The adhesive tape of claim 40, wherein the tape has an effectivethermal conductivity of at least 0.4 W/m-K as measured according to ASTMD 5470-95.
 48. The adhesive tape of claim 40, wherein the tape hasthickness of less than 300 μm.
 49. An assembly comprising the adhesivetape of claim 40 bonded between two substrates, optionally wherein thetape provides thermal conductivity between the two substrates.